The present invention relates to an image capturing apparatus, and specifically relates to an image capturing apparatus, which is provided with a image sensor having a first photo-electronic conversion characteristic, a second photo-electronic conversion characteristic being different from the first photo-electronic conversion characteristic and a third photo-electronic conversion characteristic exhibiting a transient characteristic between the first and second characteristics.
Recently, a car called an Advanced Safety Vehicle (hereinafter, referred to as the ASV, for simplicity), for which various safety measures are positively taken into account, have been developed in various kinds of technical fields. Among other things, measures for contributing the safety drive by employing images captured by an on-vehicle camera have been considered intensively (for instance, refer to Non Patent Document 1, Press Release of Nissan Motor Co. Ltd., titled “Nissan Motor Co. Ltd. achieves a first installing of a lane keeping support system into a new model of Cima in the world”, http://www.nissan-global.com/GCC/Japan/NEWS/20001228—0.html: retrieved at Feb. 22, 2006).
Since it is required for the on-vehicle camera to capture such a subject that its luminance range is very wide and its luminance tends to abruptly fluctuate in such a range of environments from a direct sunshine to an inside of tunnel, the expansion of the luminance range sensible for the image sensor, namely, the expansion of the dynamic range of the image sensor, has been considered as one of the major subjects to be attained. Further, since a traveling speed of the vehicle is apt to become high, the captured images would stream at the normal frame rate (for instance, when the traveling speed of the vehicle is 100 Km/hour, the vehicle runs 93 cm during one frame period of a normal image capturing operation at 30 frames/second). Accordingly, to securely capture the subject concerned, it is required for the on-vehicle camera to conduct the image capturing operation at a high frame rate (for instance, to such an extent of 10 times of the normal frame rate).
To cope with the expansion of the dynamic range of the image sensor mentioned in the above, there has been well known the logarithmic conversion type image sensor (hereinafter, referred to as a logarithmic image sensor, for simplicity), which includes a solid-state image sensor constituted by photo-electronic conversion elements, such as photo diodes, etc., aligned in a matrix pattern and logarithmic conversion circuits, each provided with MOSFET (Metal Oxide Semiconductor Field Effect Transistor), etc., so as to logarithmically convert the intensity of incoming light to the output electric signals according to its established output characteristic, by using the sub-threshold characteristic of the MOSFET (for instance, set forth in Patent Document 1 (Tokkaihei 11-298798, Japanese Non-Examined Patent Publication)).
However, with respect to the logarithmic image sensor set forth in Patent Document 1 (Tokkaihei 11-298798), it exhibits considerable after-image when the illuminance on the image capturing surface changes from a high level to a low level, namely, the luminance of the subject changes from bright to dark. For instance, when capturing a moving image at night, a bright light, such as a headlight of a car, etc., leaves a long tail behind it as if a ghost light ball were flying in the black sky, resulting in difficulty of recognizing the image concerned.
To overcome the abovementioned drawback, Patent Document 2 (Tokkai 2002-77733, Japanese Non-Examined Patent Publication), for instance, sets forth the linear-logarithmic conversion type image sensor (hereinafter, referred to as a linear-logarithmic image sensor, for simplicity) that makes it possible to automatically change the output characteristic inherent to the solid-state image sensor, namely, the linear operating state in which intensities of the incoming light are linearly converted to the output electric signals, and the logarithmic operating state mentioned in the above, to each other (hereinafter, referred to as a linear-logarithmic characteristic, for simplicity). According to the linear-logarithmic image sensor mentioned in the above, since the aforementioned problem in regard to the after-image can be solved, it becomes possible to improve the sensitivity of the image sensor in the low illuminance region, and as a result, it becomes possible to conduct the image capturing operation at a high frame rate.
On the other hand, in the ASV, the images captured by the on-vehicle camera are image-processed, for instance, to extract a car, an obstruction material, etc. being currently running in front of the vehicle concerned, in order to secure the safety drive. In such the case, since it takes much time to image-process the images captured by the aforementioned linear-logarithmic image sensor, in which the linear characteristic portions and the logarithmic characteristic portions are mixed with each other, due to the complexities of the image-processing operations, the linear-logarithmic image sensor is not suitable for the high-speed image-processing operations required to be conducted in the on-vehicle camera, as it is.
To cope with the abovementioned problem, with respect to the image capturing apparatus provided with the linear-logarithmic image sensor, for instance, Patent Document 3 (FIG. 11 of Tokkai 2005-348005, Japanese Non-Examined Patent Publication) sets forth the method for making the gradation characteristic over the concerned image uniform (or the same characteristic) by applying the gradation conversion processing corresponding to the linear characteristic region, and the logarithmic characteristic region to the image data representing the image concerned.
In the descriptions of Patent Document 3 (Tokkai 2005-348005), as shown in FIG. 11 of Patent Document 3, it is assumed that the linear characteristic region and the logarithmic characteristic region are completely changed to each other at the changeover point between the linear characteristic region and the logarithmic characteristic region (hereinafter, referred to as an inflection point). In reality, however, there is a region exhibiting a transient characteristic being an intermediate characteristic between the linear and logarithmic characteristics (hereinafter, referred to as a T-Log region) in the border region between them. In the normal operating condition, since the T-Log region is too narrow to take it as a problem, no problem would arise even if it is assumed that the linear characteristic region and the logarithmic characteristic region completely shift to each other at the inflection point. However, as detailed later referring to FIG. 22 through FIG. 26 of the present invention, for instance, when the image capturing operation at the high frame rate required for the on-vehicle camera should be conducted, or, for instance, when the linear characteristic region is wide in such the case that the inflection point is set at a high point of the output characteristic of the image sensor, the T-Log region is widened and it becomes necessary to conduct the gradation conversion processing for the T-Log region, which is neither linear characteristic nor logarithmic characteristic.